Long Term Evolution (LTE) is a standard for high-speed wireless communication for mobile phones and data terminals (collectively referred to here as “user equipment” or “UE”). The LTE standard is developed by the 3rd Generation Partnership Project (3GPP) standards organization.
LTE supports multiple downlink transmission modes, most of which support downlink transmission using multiple antennas. Indeed, support for the use of multiple antennas is a core feature of LTE.
In general, these multiple-antenna LTE downlink transmission modes are performed using precoding. Precoding performs a matrix transformation of v sequences of symbols x into p sequences of symbols y, where v corresponds to the number of layers being used and p corresponds to the number of transmit antenna ports being used. The number of layers being used is also referred to as the “transmission rank.” One or two data code words are transmitted per layer. A precoder matrix W is used for precoding.
The multiple-antenna LTE downlink transmission modes that employ precoding use either transmit diversity or spatial multiplexing.
In the case of transmit diversity (for example, LTE downlink transmission mode 2), a single sequence of symbols is transmitted from multiple antennas. A single code word is sent. With transmit diversity using two transmit antennas, the precoding is done using a precoding matrix W specified by the LTE standards that is based on Space-Frequency Block Coding (SFBC). With transmit diversity using four transmit antennas, the precoding is done using a precoding matrix W specified by the LTE standards that is based on SFBC and Frequency-Shift Transmit Diversity (FSTD)
In the case of spatial multiplexing, multiple layers are transmitted simultaneously from multiple antennas. Some forms of multi-antenna transmission (for example, LTE downlink transmission mode 4) make use of a codebook. A codebook is a table that comprises several predefined precoder matrices W that can be selected for use in precoding.
Cell specific reference symbols (CS-RS) are inserted in between data symbols transmitted to the UE. The UE uses the CS-RSs to generate channel state information (CSI) that is fed back to the eNodeB. This CSI includes a rank indicator (RI) and a precoder matrix indicator (PMI). The RI indicates how many layers should be used for the current channel conditions, and the PMI indicates which one of the predefined precoder matrices included in the codebook should be used for the current channel conditions.
However, the precoding schemes used with such multiple-antenna LTE downlink transmission modes are not designed for use with a specific base station or radio access network configuration.